This invention relates generally to lubricants, and more particularly to a hybrid lubricant in which microfine particles of PTFE are dispersed in an oil lubricant carrier that includes a small but effective amount of an oil-soluble organic molybdenum compound that renders the hybrid lubricant effective throughout the entire pressure range, including extreme pressures.
Even the most carefully finished metal surfaces have minute projections and depressions therein which introduce resistance when one surface shifts relative to another. The application of a fluid lubricant to these surfaces reduces friction by interposing a film of oil therebetween, this being known as hydrodynamic lubrication. In a bearing, for example, the rotation of the journal causes oil to be drawn between it and the bearing so that the two metal surfaces are then separated by a very thin oil film. The degree of bearing friction depends on the viscosity of the oil, the speed of rotation and the load on the journal.
Should the journal start its rotation after a period of rest, it may not drag enough oil to float the surfaces apart; hence friction would then be considerably greater; the friction being independent of the viscosity of the lubricant and being related only to the load and to the "oiliness" property of the residual lubricant, causing it to stick tightly to the metal surfaces. This condition is referred to as "boundary lubrication," for the moving parts are then separated by a film of only molecular thickness. This may cause serious damage to overheated bearing surfaces.
The two most significant characteristics of a hydrodynamic lubricant are its viscosity and its viscosity index, the latter being the relationship between viscosity and temperature. The higher the index, the less viscosity will change with temperature. Fluid lubricants act not only to reduce friction, but also to extract heat developed within the machinery as well as a protection against corrosion.
Though fluid film separation of rubbing surfaces is the most desirable objective of lubrication, in practice it is often unobtainable. Thus bearings built for full fluid lubrication during most of their operating phases actually experience solid-to-solid contact when starting and stopping.
Typical solid lubricants are soft metals such as lead, layer lattice crystals such as graphite and molybdenum disulphide, and crystalline polymers such as "FLUON" (polytetrafluoroethylene, or PTFE). Integral bonding of these solid lubricants to the surfaces of the bodies to be lubricated is desirable for good performance.
Under severe operating conditions usually encountered in automotive transmissions and in internal combustion engines, hydrodynamic or fluid lubrication is inadequate to minimize friction and wear; for fluid film separation of the rubbing surfaces is not possible through all phases of operation. Hence, the ideal lubricant for engines or other mechanisms having moving parts is one combining hydrodynamic with solid lubrication. In this way, when adequate separation exists between the rubbing surfaces, a protective fluid film is interposed therebetween; and when these surfaces are in physical contact with each other, friction therebetween is minimized by interposing solid lubricants between these surfaces.
In theory, one can best approach this ideal by lining the rubbing parts of engines with solid lubricant layers which are integrally bonded thereto, concurrent use being made of a lubricating oil which functions not only to provide hydrodynamic lubrication but also to cool the rubbing parts. In addition, the oil may carry synthetic organic chemicals to carry out other functions to counteract wear and prevent corrosion.
The practical difficulty with attaining this ideal is that parts coated with solid lubricants, such as a PTFE layer, are very expensive and therefore add considerably to the overall cost of the engine. Moreover, in PTFE-coated parts which operate under rigorous conditions, the solid lubricant layers bonded thereto have a relatively short working life, so that is is not long before the only lubricant which remains effective in the engine is the fluid lubricant.
In order to provide lubricating activity that has both solid and fluid components, my prior U.S. Pat. No. 4,127,491 and the above-identified related applications disclose a modified oil lubricant suitable for an internal combustion engine provided with an oil filter as well as for many other applications which call for effective lubrication throughout all phases of operation. This modified lubricant is constituted by major amounts of a conventional lubricating oil intermingled with minor amounts of an aqueous dispersion of polytetrafluoroethylene (PTFE) particles in the sub-micronic range in combination with a neutralizing agent which stabilizes the dispersion to prevent agglomeration and coagulation of the particles. The modified lubricant is therefore capable of passing through the oil filter without separating the solid particles from the oil in which it is dispersed.
This modified lubricant has many significant advantages; for, as indicated in my prior patent, it reduces wear and thereby prolongs engine life; it makes possible a sharp reduction in the emission of pollutants and also effects a significant improvement in fuel economy, the last factor being of overriding importance in a fuel-short world.
A hybrid lubricant of the type disclosed in my earlier-filed patent applications is most effective as a friction reducer when the friction arises from contact pressure between rubbing metal parts that is spread over a broad area, such contact pressure arising, for example, at the interface between a shaft and a sleeve bearing within which the shaft rotates. Though friction encountered in internal combustion engines largely falls into the broad contact category, the engine also has high points in various regions wherein the friction is concentrated at point contact areas. Because of the resultant extreme pressures, these point contact areas are difficult to lubricate effectively and run relatively hot. A solid PTFE lubricant layer is difficult to maintain on point contact surfaces and a hybrid lubricant of my prior type is therefore of limited effectiveness under extreme pressure-point contact conditions in an engine.
As pointed out in my copending application Ser. No. 158,329, when the colloidal PTFE particles in the hybrid lubricant are brought into contact with rubbing metal surfaces formed of aluminum or other metals having porous oxide surfaces, the PTFE particles are impregnated into the granular interstices and voids to create thereon a thin PTFE layer which renders these surfaces extremely slippery.
However, steel and other metals having non-oxidized surfaces resist impregnation by the PTFE particles in the hybrid lubricant. Where the rubbing surfaces in the engine are aluminum against steel, the PTFE layer developed on the aluminum affords the necessary solid lubricant at the interface thereof. But when the surfaces are both steel, a hybrid lubricant of the type disclosed in my prior applications and patents is then less effective as a friction reducer, particularly when point contact conditions are involved.